Atomic layer deposition (ALD), also known as atomic layer epitaxy, is a process for depositing highly uniform and conformal thin layers of a metal on a surface. The surface is exposed to vapors of the metal precursor and a reducing agent. Such films have a wide variety of applications in semiconductor microelectronics and optical films. The conventional ALD process, which uses a two-step procedure, is described by M. Ritala and M. Leskela in “Atomic Layer Deposition” in Handbook of Thin Film Materials, H. S. Nalwa, Editor, Academic Press, San Diego, 2001, Volume 1, Chapter 2.
In a typical two-step ALD process, there is a self-limiting adsorption of the metal complex to the surface that is controlled by the interaction of the precursor with the substrate in a thermal degradation step. The loss of the ligand is induced thermally, as the metal surface has no functional groups to induce ligand loss chemically. The metal precursor is desirably stable enough to be transferred into the reaction chamber, and reactive enough to undergo a transformation at the substrate surface.
In a related ALD process, the substrate contains functional groups that react chemically with at least one ligand on the metal-containing precursor. For example, a typical process used to prepare thin, conformal Al2O3 films uses a substrate with hydroxyl groups. The substrate is contacted with Al(CH3)3, which reacts with the surface hydroxyl groups to form an adsorbed Al—O complex and liberated methane. When the surface hydroxyl groups are consumed, the reaction stops. Water is then contacted with the Al—O complex on the surface to generate an aluminum oxide phase and additional hydroxyl groups. The process is then repeated as needed to grow an oxide film of desired thickness. The deposition rate of the Al(CH3)3 is controlled by the number of surface hydroxyl groups. Once the hydroxyl groups are consumed, no additional Al(CH3)3 can be adsorbed to the surface.
In other known ALD processes for the deposition of metal films on substrates of interest, there may be no reactive group on the substrate surface to initiate the type of self-limiting reaction that is seen in the Al2O3 case. For example, in the deposition of a metal barrier layer on a tantalum nitride substrate, the self-limiting adsorption is achieved through the thermal decomposition of the precursor. Therefore, the precursor is preferably designed to have the volatility and stability needed for transport to the reaction chamber, but also the reactivity to undergo clean thermal decomposition to allow a metal complex to chemisorb to the substrate surface. Often, these processes produce films contaminated with fragments from the metal ligands degraded during the thermal deposition.
In an ALD process for depositing ruthenium films, the substrate in a reaction chamber is exposed sequentially to a ruthenium precursor and a reducing agent or an oxidizing agent introduced alternatively. The substrate is exposed to the first reactant, which is a ruthenium precursor that is chemisorbed onto the surface of the substrate. Excess reactant is removed by purging the reaction chamber. This process is followed by the exposure of the chemisorbed complex on the substrate to the second reactant, usually a reducing agent, which reacts with the metal complex to produce the ruthenium film. The second reagent removes the organic ligand from the metal precursor and reduces the metal ion to its elemental state. The reaction chamber is again purged to remove excess reducing agent. The cycle can be repeated, if needed, to achieve the desired film thickness. (U.S. Pat. No. 6,617,248 and WO2004/035858)
U.S. Pat. No. 6,824,816 has disclosed the atomic layer deposition of ruthenium from ruthenium precursors such as bis(cyclopentadienyl)ruthenium, bis(ethylcyclopentadienyl)ruthenium, tris(2,4-octanedionato)ruthenium, tris(2,2,6,6-tetramethyl-3,5-heptanedionate)ruthenium, and bis(pentamethylcyclopentadienyl)ruthenium. In this process, the precursor is deposited as no more than a single monolayer on a substrate surface, and then the deposited precursor is reacted with a second reactant gas comprising oxygen to give a Ru metal layer. The sequence of deposition and reaction steps can be repeated to provide thicker metal layers. The growth temperature of the metal thin film is approximately 200-500° C., preferably 300-360° C. One disadvantage of the process is that oxygen is not compatible with some barrier layers that are used in the manufacture of electronic devices. Another disadvantage is that the deposited Ru metal film may contain unacceptably high levels of contaminants derived from the oxidation of the precursor ligands.
US 2004/0092096 discloses a method for improving the adhesion between a diffusion barrier film and a metal film, by creating a monolayer of oxygen atoms between the diffusion barrier film and the metal film. Suitable metals include Cu, Al, Ni, Co and Ru. In one embodiment, the monolayer is created by exposing the diffusion barrier film to an oxygen-containing reactant and then depositing the metal film via CVD, ALD, PVD or sputtering.
There is a need for a process for the formation of oxide-free ruthenium-containing films that can be run at relatively low temperatures and that can provide high quality, uniform films of high purity. Such a process would allow the electronics industry to take advantage of the desirable properties of Ru, such as its ability to serve as a seed layer for copper electrodeposition and its ability to be patterned by etching.